1. Field of the Invention
The present invention relates to an optical encoder having a reflection mechanism using an annular diffraction grating for detecting information relating to the position or the angle in an industrial measuring apparatus or the like.
2. Description of the Related Art
FIG. 1 is a perspective view illustrating the configuration of a conventional linear encoder. In FIG. 1, a light beam from a semiconductor-laser light source 1, serving as a coherent light source, is divided into polarized components by a polarizing beam splitter 3 via a collimator lens 2. A P-polarized light beam passing through the polarizing beam splitter 3 is projected onto a diffraction-grating portion on a scale 4 with an angle θ after being propagated on an optical reflecting surface, is emitted as a +first-order diffracted light beam by being reflected, is returned to the original optical path by a reflecting optical element 6 via a ¼-wavelength plate 5, and is finally returned to the polarizing beam splitter 3 by being subjected to +first-order diffraction.
An S-polarized light beam reflected by the polarizing beam splitter 3 is projected onto the diffraction-grating portion on the scale 4 with an angle θ after being propagated on the optical reflecting surface, is emitted as a −first-order diffracted light beam by being reflected, is returned to the original optical path by a reflecting optical element 6 via a ¼-wavelength plate 5, and is returned to the polarizing beam splitter 3 by being subjected to −first-order diffraction.
Since the ¼-wavelength plate 5 is inserted in the optical path of each of the diffracted light beams, the orientation of polarization is shifted by 90 degrees during the back and forth movement, so that the +first-order diffracted light beam and the −first-order diffracted light beam are returned to the polarizing beam splitter 3 as an S-polarized light beam and a P-polarized light beam, respectively. Accordingly, the +first-order diffracted light beam is reflected by the polarizing beam splitter 3 and the −first-order diffracted light beam passes through the polarizing beam splitter 3, and the two light beams are emitted in a state in which the wave surfaces of the two light beams are superposed. Then, the superposed light beams are converted into a linearly-polarized light beam, in which the orientation of polarization changes based on the phase difference between the two light beams, while passing through a ¼-wavelength plate 7. The light beam is then divided into two light beams by a non-polarizing beam splitter 8 provided behind the ¼-wavelength plate 7. Only light beams having specific orientations of polarization are extracted by polarizing plates 9a and 9b, and light/dark signals are obtained in photosensors 10a and 10b. 
Since the phases (timings) of the light/dark signals are provided by deviations in the orientations of polarization of the polarizing plates 9a and 9b, the phase difference between the light/dark signals is set to 90 degrees by shifting the orientations of polarization of the polarizing plates 9a and 9b by 45 degrees.
A refractive-index-distribution-type lens optical system is used as the reflecting optical element 6, whose length is selected so as to condense an incident parallel light beam at an end surface. A reflecting film is coated on the end surface.
Such an optical element is called a cat's eye, and has the property of guiding an incident light beam in the entirely opposite direction. In general, the above-described encoder has the properties that, when the wavelength of the semiconductor-laser light source 1 changes, the diffraction angle changes to shift the optical path and to change the angle between two light beams to be subjected to interference. As a result, the state of interference changes.
Furthermore, if the relative alignment between the scale 4 and the detection head unit shifts, the optical path is also shifted:
However, by using the above-described reflecting optical element 6, the light beam moves with an original angle even if the diffraction angle changes, so that the path of the rediffracted light beam does not change. As a result, stable measurement can be performed.
However, in the above-described conventional approach, the reflecting optical element 6 must have a size of about 5 mm. Since it is necessary to project the light beam substantially perpendicularly in order to obtain a predetermined performance, the location to dispose the reflecting optical elements 6 must generally be determined in accordance with the diffraction angle. In addition, since the reflecting optical elements 6 are obliquely disposed in the space, reduction in the size of the entire encoder is limited.
When the scale 4 is a rotary encoder, a radial diffraction grating is used. In this case, if the light beam is not projected onto a central portion of the cat's eye, the location irradiated by the returned light beam is slightly shifted when the returned light beam is reprojected onto the diffraction-grating scale 4.
As a result, the phenomenon that the orientation of the rediffracted light beam is shifted occurs. The influence of this phenomenon is larger as the pitch of the grating is smaller to the order of micrometers and the diameter of the radial-grating scale (the diameter of the disk) is smaller. In a type in which the scale 4 and the detection unit are separated, this influence greatly appears as an alignment error due to errors in the gap between the scale 4 and the detection unit, the angle of installation of these units, and the like. Accordingly, a system using a cat's eye has a limitation in the use of a finer radial-grating scale.
A grating interference encoder of this type adopts a micrometer-order fine scale. By causing two light beams obtained as a result of diffraction by this scale to interfere with each other, a encoder having a much higher resolution than a geometrical-optics-type encoder is obtained.
This encoder adopts a configuration of generating an interference pattern by synthesizing the wavefronts of two diffracted light beams. Since the encoder is constituted as an interference optical system, it is necessary to very precisely process respective optical elements and very precisely dispose these elements Particularly in the case of an assembled encoder in which a scale unit and a detection-head unit are separated, since the user must mount the scale unit and the detection-head unit on a motor, a stage or the like, difficulty in the mounting operation causes problems.